Why does the Torah write, "and from the animals that were not Tahor," and not simply "and from the Tamei animals"?
The Gemara in Pesachim 3a explains that it is to teach us the importance of clean speech. 1
Since 'Tamei' has unpleasant connotations, and the Torah therefore adds eight letters to avoid using it.
Here, the Torah elaborated to avoid the word "Tamei." Yet elsewhere, the Torah does use the word " Tamei"!
Rashi (Pesachim 3b): One should teach concisely. The Torah elaborated once, to teach the preference for a clean expression.
Me'iri (Pesachim 3a): Here it has a negative connotation, i.e. species that interbred. (In the days of Noach there was no difference between Tahor and Tamei.)
Peninim mi'Shulchan ha'Gra: Elsewhere, the Torah wants to debase the Tamei species, so that we will avoid eating them. Here there was no need to do so. 1
Pesachim 3b answered only that it is better to teach concisely! According to the Vilna Gaon, elsewhere, even if both expressions have equal length, it is better to say "Tamei"! (PF)